1. Field of the Invention
The invention relates to a process for cleaning, drying and simultaneously hydrophilizing a semiconductor wafer, the drying taking place with rotation of the semiconductor wafer.
2. Background Art
Semiconductor wafers which serve, for example, as substrates for the production of microelectronic components, usually silicon wafers, are cleaned by wet chemical processes after polishing, coating (for example by epitaxial deposition) or thermal treatment steps (“annealing”) or before high-temperature process steps. The aim of cleaning is to remove contaminants of the semiconductor wafers, for example metals such as copper or organic substances, and also particles adhering to the wafer surface. All these should be removed as completely as possible, since the contaminants lead to problems during subsequent production of components, as two examples, an inhomogeneous growth of gate oxides or an inhomogeneous deposition of polysilicon gates.
A number of processes for cleaning and drying semiconductor wafers are known in the prior art:
U.S. Pat. No. 5,714,203 discloses a process in which a semiconductor wafer immersed completely into a bath comprising a hydrogen fluoride solution (hydrofluoric acid) is slowly withdrawn from the bath into an ozone-containing atmosphere. The ozone from the atmosphere dissolves at the liquid surface and thus reduces the surface tension thereof, which makes it easier for the liquid to flow away from the wafer surface. At the same time, the wafer surface is oxidized by the ozone-containing atmosphere. The wafer is therefore hydrophilic after drying. It was established in U.S. Pat. No. 6,451,124 that the semiconductor wafer is permitted to be withdrawn from the hydrofluoric acid at a speed of only 0.01 to 15 mm per second, and preferably 0.01 to 0.5 mm per second, in order actually to achieve a dry wafer surface. Given a wafer diameter of 300 mm and a speed of 0.5 mm per second, this procedure alone requires 10 minutes. The wafer must be fully dry since inhomogenities are otherwise observed on the wafer surface, for example so called “watermarks”.
For some years cleaning processes for individual wafers have also been increasingly employed, in which a semiconductor wafer is caused to rotate rapidly about its center axis and is firstly cleaned with one or more liquids, then rinsed with deionized water and dried. The liquids are applied to the rotating semiconductor wafer and accelerated towards the edge of the wafer by the centrifugal force, such that the liquids flow away outwards and form a generally whole-area thin film on the wafer surface. During subsequent drying with further rotation of the semiconductor wafer, for example with addition of a vapor that reduces the surface tension of the liquid film (e.g. isopropanol), the liquid film completely flows away afterwards. Processes of this type are described in US2004/0103915A1 and EP0905747A1. Individual-wafer cleaning processes of this type are distinguished by a lower consumption of chemicals.
It is also known to use hydrofluoric acid as a cleaning liquid in such devices. Thus, U.S. Pat. No. 6,869,487 B1 describes a process in which a semiconductor wafer is cleaned with a liquid with rapid rotation, the liquid containing dissolved ozone which was either dissolved previously in the liquid or originates from an ozone-containing process atmosphere. A hydrophilic surface can be achieved by ozone furthermore being added to the process atmosphere after the supply of liquid has already been stopped. However, care should be taken to ensure that a closed liquid film is always present on the wafer surface as long as it is in contact with ozone. The semiconductor wafer is subsequently rinsed with deionized water and dried. The drying is effected, for example, by centrifugal drying with addition of isopropanol vapor.